For many applications, for example, in industrial and in traction applications, such as in the railroad field, energy is exchanged between connected loads and generators by means of AC voltage and alternating current, and DC voltage and direct current. In many applications, a different fixed frequency or variable frequency is specified. For example, not only is energy passed from the energy generator to the load, but also in the opposite direction, and converter circuits can be used for this purpose.
A converter circuit for conversion of an AC voltage to a second AC voltage is specified, for example, in DE 198 27 872. In this document, the converter circuit comprises an input-side rectifier and a capacitive energy storage circuit and inverter, which are connected downstream from the rectifier on the output side. On the output side, the inverter is connected to the primary side of a transformer. A capacitance is in each case connected in series with the primary and secondary windings of the transformer. Furthermore, the secondary side of the transformer is connected to a further inverter, with a further capacitive energy storage circuit connected downstream therefrom. The converter is controlled such that an appropriate AC voltage is produced by clocking the DC voltage at the input-side inverter, by means of the capacitive energy store, the capacitances connected in series with the transformer and the stray inductance of the transformer, on the output side to the capacitive energy store.
In the converter circuit mentioned above, despite the existing resonant circuit, switching losses can nevertheless still occur in the power semiconductor switches in the converter, thermally loading these power semiconductor switches heavily. In consequence, the power semiconductor switches age correspondingly quickly, and the failure rates of the power semiconductor switches rise with the operating life of the converter circuit. High availability of the converter circuit, as can be specified for traction applications, is then no longer ensured.
By way of example, further converter circuits are known from U.S. Pat. No. 6,344,979, which discloses a DC-DC converter circuit having an LLC intermediate resonant circuit connected to the primary winding of the transformer. The resonant capacitance therein is connected in series to the first resonant inductance, and the first resonant inductance is connected to a first end of the primary winding of the transformer. The second resonant inductance is connected in parallel with the primary winding of the transformer, that is to say it is connected on the one hand to the first end of the primary winding and the first resonant capacitance, and on the other hand to the second end of the primary winding of the transformer.
DE 10 2005 036 806 discloses a circuit for a welding current source having resonant converters which are arranged in parallel and are connected to a load circuit via a respective transformer. The secondary sides of the transformers are connected in series. The individual resonant circuits are formed by a capacitance and an inductance in series with the primary side of the transformer, and a capacitance in parallel with the secondary side of the transformer.
DE 102 04 219 discloses a converter system having a plurality of partial converter systems for feeding a load. Each partial converter system has a transformer which is connected on the input side directly to a medium-frequency DC voltage inverter.